This invention relates to the formation of contact electrode bumps in structures such as integrated circuit wafers and mounting substrates.
High volume, fast handling and economical packaging systems are being developed for packaging microelectronic components to meet a growing demand for such components and systems of such components at a reasonable cost. One of the component mounting techniques, as a part of one of these packaging methods, is the so called "face-down" bonding technique. In this technique, contact electrodes in the form of small metallic bumps are provided on selected portions of a metallization interconnection network, usually, where electrical connections to external components are desired. This metallization interconnection network is provided in the component to be mounted, typically an integrated circuit chip. The component is to both be mounted by and to make electrical connection through these bumps.
Integrated circuit chips having these bumps thereon are placed with the network side facing a surface of a mounting substrate and placed such that the bumps are in contact with conductive lead portions provided on the substrate to which the bumps are to be attached. Next, heating the bumps causes them to flow and thus come into intimate contact with these conductive lead portions of the substrate so that upon cooling the desired bond results. It is therefore required that (1) the metal used for construction of the bumps and (2) the materials and structure with which this bump metal is to interface in a bond together result in bumps having sufficient cohesiveness in the molten state to provide support to the chip -- i.e. so that the bumps do not collapse when molten. Such collapse would bring the chip into close proximity with or directly onto the substrate with the bump metal spread about so far as it would flow. A standoff bump not used in an intimate bond with the substrate conductive leads and perhaps not formed on the interconnection network can be used to keep the chip separated from the substrate, though it cannot prevent molten bump metal from flowing. Flowing of the bump metal can be prevented whether a standoff bump is used or not, by use of a glass dam on the substrate, as an example, surrounding the point of which a bond is intended to be made.
Another packaging method also has metallic bumps usually formed on a metallization interconnection network provided with a component to be packaged, again typically an integrated circuit chip, the bumps to be provided at selected portions of the network where contact with external components are desired. Contact with external components is again provided through connection with conductive lead portions provided on a mounting substrate. The chip mounting technique in this packaging method, however, differs from the "face-down" technique in that the chip face opposite the face having the interconnection network provided thereon is to also be mounted on a surface of the mounting substrate. This requires a lead frame having conductive leads therein with these conductive leads bonded to the bumps to provide leads for connection to the conductive lead portions provided on the substrate. This bonding technique may be termed the lead frame mounting technique.
The bumps in this bonding technique are not intended to flow appreciably when heat is applied to them for the purpose of their being bonded to the lead frame conductive leads. Rather, an eutectic bond is to formed between the bumps and the conductive leads requiring (1) that the bump metal and (2) the lead frame conductive lead metal be able to form a satisfactory eutectic bond. Since no appreciable flow is to occur there need be no provision to prevent a flow of the bump metal down the lead frame conductive lead.
In the first mounting technique noted above, it was indicated that some provision must be taken with respect to the substrate and the bump metal to prevent the bumps, which are intended to flow, from flowing any great distance across the substrate. This need to restrict the flow of molten bumps exists, as set out above, because unrestricted flow would cause the bumps to diminish and leave little to provide a standoff distance between the integrated circuit chip and the substrate. Even if a standoff bump is provided, unrestricted flow of the bump metal would result in no intimate electrical contact or an improper contact between chip and substrate.
As the above indicates, the choice of a packaging method and so of the component mounting technique (1) limits the kinds of metals which are satisfactory for making the contact electrode bumps on, for instance, a component metallization interconnection network and (2) limits the kinds of materials and/or structure which can be used in or near the network and the item to be bonded to at the points the contact electrode bumps are to be formed or bonded, respectively. Again, for example, an interconnection network is taken as provided on an integrated circuit chip and the chip is to be mounted on and to make electrical contact to a mounting substrate. In the "face-down" technique, the contact electrode metal must be capable of flowing and usually cannot be allowed to wet much of the surface in either its immediate area on the integrated circuit chip or in the immediate area of the intended bond on the conductive lead portion of the substrate. When this is the case the surface tension of the molten metal comprising most of the bump serves to give the molten bump enough structural integrity to maintain a standoff distance between the chip and the substrate.
A well-known metallurgical system for a bump used in the "face-down" mounting technique is provided in four layers on a selected portion of the interconnection network. The selected portion is usually available as an exposure of the network through an opening in an insulating overlayer. The interconnection network is usually constructed of aluminum.
To provide the metallurgical system, a metal mask is placed on the insulating overlayer with the mask having an opening therein that is concentric with but larger than the opening in the insulating overlayer. By use of evaporation deposition techniques, a layer of chromium is deposited on the aluminum through the opening in the metal mask and the opening in the insulating overlayer. This is followed by evaporating copper onto the chromium which in turn is followed by evaporating gold onto the copper. These three metal layers are for the purposes of sealing, preventing unwanted intermetallic compounds from forming and to provide properly adhering films to the various interfaces. The total thickness of these three layers is typically 10,000 A or less. It is important that not too much gold be provided because it forms unwanted intermetallic compounds with the tin in the solder which is to be next evaporated onto the gold. Solder then is evaporated onto the gold and forms the main body of the bump usually being several mils in height.
The use of the lead frame mounting technique, however, generally prevents the use of solder as a major component in the body of the bump. Typically, solder is not used to and does not form a eutectic bond with another metal being joined therewith in such a way as to include, as a constituent, that other metal in the metallurgical bond to be formed. Rather, solder is used in joining with another metal because of its adhesion properties to the surface of the other metal when the solder solidifies from the molten state in which state it wetted the surface of the other metal. If, however, a lead frame is brought into contact with a molten solder bump the result will be that the solder will flow along the lead frame lead in such quantity as to greatly diminish the bump. Further, applying pressure through the lead frame lead to the molten solder bump is unavoidable in bringing the lead frame into contact with the bump. This pressure on the molten solder bump will overcome the surface tension of the molten solder bump giving the bump the necessary cohesiveness to prevent it from flowing and so flow across the surface on which the bump is provided will occur worsening the molten state flow problem.
Thus, solder typically isn't a satisfactory metal for a major portion of a bump body where a eutectic bond is to be formed with a lead frame lead made of a metal commonly used for such leads. The use of a solder bump in a molten state for attaching a lead frame lead would be difficult without making undesirable alterations in the lead frame lead itself and/or in the surroundings of the point on the surface where the bump is provided.
Use of eutectic bonding for the lead frame mounting technique requires that the metal for use in the major portion of the bump body and the metal to be used for the conductive leads in the lead frame be capable of forming a eutectic bond at pressures and temperatures not too extreme considering the chip and the substrate must thereafter by functional. The eutectic temperature, which will be below the melting point of either metal involved, must be substantially higher than that at which the bond to the substrate conductive leads is formed at the opposite end of the lead frame lead. This latter bond to the substrate conductive lead is usually a solder bond. These criteria assure that the metal comprising the major portion of the bump electrode can form eutectic bond with the relatively small amount of metal in the lead frame lead without the bump appreciably losing its structural and shape characteristics as the eutectic temperature is reached and further assures that the bond will not be degraded by the formation of a bond to the substrate conductive leads at the opposite end of the lead frame lead. One additional desired feature for the metal which is to comprise the major portion of the bump body is that it be relatively inert so as to tend to not form undesirable compounds which may effect the integrity of the bonds or otherwise effect materials surrounding the bond.
A satisfactory combination of metals for the lead frame mounting technique are gold for the major portion of the bump body and tin which can be plated on a lead frame conductive lead of copper. This combination of metals being satisfactory, the same layer pattern of metals could be used to construct bumps in the lead frame mounting technique as are used in the "face-down" mounting technique up to the deposition of the solder, these being chromium on the aluminum interconnection network, then copper and finally gold with the gold present in much greater quantity than in the "face-down" technique to provide the desired bump height. Other metals can be used in place of the copper and the chromium. Further, combinations of metals other than gold and tin are also possible.
Since the same layer pattern of metals may be placed on the aluminum interconnection network, it would seem that the same deposition technique could be used to place them there, i.e. evaporation. However, the use of evaporation to deposit an expensive metal in quantity, such as the gold here, is uneconomical because such a small fraction of the metal evaporated reaches the position desired for deposition but is rather deposited in many places in the deposition chamber since the evaporated metal fills the chamber with little directivity.
Plating methods have also been used to construct contact electrode bumps on interconnection networks. However, in known plating methods a metal mask is not used. Rather, the mecessary layers of the metals, the same chromium, copper and gold pattern, are blanket evaporated over the entire surface of the insulating layer covering the interconnection network and over the exposed selected portions of the interconnection network. An undesirable result of this is that on cooling, particularly in the case of integrated circuits, the rapid contraction of the now condensed metals tends to cause microcracking at various locations of the insulating layer provided over the interconnection network which leads to yield loss is subsequent operations.
After the blanket evaporation, a photoresist layer is placed over the last evaporated metal and openings are provided in the photoresist across from the selected portions of the interconnection network where bumps are desired to be placed on the last evaporated metal. A gold plating process over the result is used but gold will adhere and plate only to the exposures of the last evaporated metal in the photoresist layer openings. The photoresist is then removed as are those large portions of the several layers of evaporated metals which are not serving as a base for a bump. Removing these evaporated metals requires an undesirable multiplicity of etching steps. If the microcracking noted above has occurred at points so as to expose portions of the interconnection network below, one or more of the etching steps may also etch the aluminum used in the interconnection network.
At this point a sintering heat treatment in dry nitrogen is required to cause the interconnection network material, usually aluminum, to diffuse through both its own oxides and through any insulating layer material residue formed on the selected portions to be sure a good contact is made with the overlaying evaporated metal layers if a relatively small electrode resistance is desired. This sintering is required because cleaning methods are limited without the use of a metal mask to protect surfaces because of the resulting degradation of surface characteristics of those surfaces not involved in supporting the bumps.
The lead frame component mounting technique, particularly for integrated circuits, is a quite desirable technique when the circuit operation of the components as mounted is considered because the component face not having the bumps thereon is mounted directly on the substrate. This provides, as a natural step of the technique, much better thermal dissipation for heat generated by the component during circuit operation than does the "face-down" mounting technique where much of the heat dissipation would be through the bumps alone unless further measures were undertaken. Thus it is desired to have an economical and effective process for providing the contact electrode bumps on the component for use in the lead frame component mounting technique.